Feature

All Energy 2012 preview: Big blades - the Vestas approach


George Marsh

The new-generation of wind turbines that will operate offshore require very large rotors. George Marsh takes a look at the Vestas approach.

Renewable Energy Focus is proud to be the official "Innovation in wind technology" media partner for this year's All-Energy event in Aberdeen, taking place on 23-24 May. The All-Energy Exhibition & Conference is the UK's largest renewables event devoted to all forms of clean and renewable energy. It is being held at an important time as the UK continues to assume a dominant role in the offshore wind sector and many companies look to the immense supply chain opportunities this brings.

WITH A blade length of 80 metres, the rotor for Vestas' latest V164-7MW offshore wind turbine, now in development, will sweep an area 50% greater than the London Eye.

With that mind-boggling vision in mind, the Danish firm is already well ahead in getting the huge production facilities that will be required ready.

Indeed, its new blade prototype and test facility was recently completed on the banks of the River Medina near Newport, Isle of Wight, UK.

Spanning 19,000sq ft, the scale of the £50 million facility is impressive. Indeed, it is “the world's largest test facility for wind turbine blades” – pointed out UK Energy Minister Chris Huhne on a visit to the site in August.

Built on a 15-acre site, once an old tide mill and later a cement works, two massive 170 metre halls now stand ready to accommodate the largest blades envisaged. One hall will be for fabricating prototype blades, while the other will be used to test them.

Each hall is 50 metres wide and is 36 metres high. The height is needed to allow for large flapwise bending of blades during structural and fatigue testing. Large full-span steel overhead beams carry Konec travelling cranes for maneuvering blades, jigs and tools into required positions. Foundations for test rigs, extending to 35 metre depths for the most critical bearing test rigs, are isolated from the rest of the building structure.

Maintaining consistent quality in composite fabrications becomes harder with the larger blade sizes. It is also more critical, both because of the potential for loss when a blade is scrapped due to a manufacturing issue and because of the loss of revenue consequent on a blade failure in service. Clearly, if blades are to remain in use for 20 years or more in the harsh offshore environment, only the highest structural and manufacturing standards will suffice.

“We wanted a facility where we could not only verify the mechanical and other properties of rotor blades, but also the critical-to-quality processes needed to manufacture those blades,” says Rob Sauven, managing director of Vestas Technology, UK. “In fact, we wanted a mini factory. We'll be developing our blades and fabrication processes to Lean 6-Sigma criteria and to achieve technology readiness at this level we'll need to prove things out with exhaustive tests and statistical rigour.”

There is also a three-storey office block at the facility, providing working accommodation for its 150 strong workforce of designers, project managers, administrators and ancillary functions [This includes around 50 people who were re-employed by Vestas, having previously worked at its nearby factory which had made smaller blades but ceased manufacturing some time ago].

The carbon factor

Engineers are well aware that blades are literally ‘up front' for optimization: a small improvement in the technology has a more than proportionate benefit in turbine efficiency overall. However, achieving that benefit requires them to balance low weight and aerodynamic requirements, which favour slim precisely shaped blades, against structural performance.

Carbon composite has come to the rescue, enabling engineers to develop large blades that are strong and stiff, while at the same time light and aerodynamic. Each V164 blade relies on a single carbon main spar for stiffness, plus other structural elements, which Vestas is keeping under wraps, to support advanced moulded skins that make up the blade envelope and confer the required aerodynamic shape.

Exploiting the stiffness of carbon in this way means the amount of pre-bend that has to be built into the blade (to avoid the potential for tower strikes in strong winds) can be minimised. This in turn avoids the adverse cost and transportation implications of having blades with high pre-bend built in.

For commercial reasons, Vestas is releasing few design details. Indeed, some of the details are yet to be finalised. Sauven stresses that the company is material and technology agnostic, using whatever provides the best answer in a given situation. “We use a whole range of techniques, not only pre-preg and resin infusion,” he declares. “The choice depends on the sizes of the blades and the manufacturing volumes envisaged.”

This is seen in the case of materials. For example, plastics have superseded the wood that Vestas once favoured as its base core material. Personally, Sauven still rates balsa, birch and other woods highly but concedes that concerns over resource sustainability and repeatable quality have seen wood give way to synthetics such as PET for structural core. Nevertheless, this is not yet the perfect solution and so the international research teams of Vestas are investigating the use of sustainable bio-substitute materials.

The global reach of Vestas is central to its R&D efforts. Sauven himself has five engineering teams at his disposal and, in prototyping the V164 blade, he can enlist expertise from centres in Denmark, the US, India, China and Singapore. Currently, Vestas has more than 300 engineers focusing on blades, including the 200 plus on the Isle of Wight.

The company also benefits from close links with academic institutions such as Bristol University and with the UK's National Composites Centre (NCC). Networking with suppliers and other manufacturers is another fruitful avenue, sharpening awareness of current practice. A unifying link between all R&D sources is a five-year technology roadmap that sets waypoints for further advancement.

Sauven believes the cure process is likely to see major advances. Research is taking place within Vestas on methods such as UV and microwave curing, he says, stressing his belief that resin chemists ultimately hold the key. “We shall see the biggest technical steps in this area,” he says. “A major challenge at scale is achieving an adequate cure in thick composite sections. To process these thicker laminates, we need resins that exotherm less during cure.” He adds: “Zero exotherm would be nice.”

While much engineering time is devoted to blade design, Sauven notes the technology needed to manufacture the blades therefore absorbs more R&D effort – up to three quarters of the total in fact. Tooling, for instance, is a leading preoccupation. Tooling for the shells of the new blades will be advanced composite, the chief drivers being shape, affordability and a match between the coefficients of thermal expansion (CTE) of the tool material and that of the composite blades.

In contrast, aluminium mandrels are specified for the strength and stiffness-critical spars, with carbon composite being placed onto them using a bespoke tape laying process. Cure will be achieved by circulating hot air or oil through internal tubing.

Business case

Sauvern emphasises that technology choices are “total” business case driven. “We look to the total, end-to-end business case to drive the right decisions,” he says. “For instance, one could address the logistical difficulties associated with large blades by making each blade in two or more sections that could be transported separately to an assembly site close to the intended wind farm and joined there. We have looked into this and in a sense we have done it in the past. After all, some of our earlier blades had pitchable tips.”

However, after assessing the costs of engineering the joints, along with the weight and structural implications, “we concluded that, for us, the business case is not yet compelling. We took into account that the 55 metre blade we produce for our 3MW V112 turbine is delivered in one piece and that this blade has proved highly popular in the market. We have been able to manage it through all stages of manufacture, delivery and erection”.

Thus sectional blades are just one of a number of technologies that are in abeyance right now as the business case for them are not yet strong enough. With manufacturing automation, the decision can go either way.

While automation is seen as essential for speedy and repeatable production of large composite items, Vestas is adopting it in a measured fashion, justifying it, or not, on a case-by-case basis. With regard to blade painting, for example, the speed and waste reduction secured by automating the process and the rapid payback achieved add up to an overwhelming case for adoption.

Less clear-cut, though, is the case for extending fibre placement automation. Vestas is adopting AFP primarily for quality reasons, though its further extension will again be business case driven. Thus, although the company's bespoke fibre placement solution may be right for blade manufacture in Europe and the Americas, the more appropriate course in lower wage economies might be to retain significant manual input. Solutions adopted have to meet regional market needs by satisfying social criteria as well as providing an appropriate balance of affordability against quality and technical performance, the firm says.

Meantime, in terms of ‘stealth' technology, used to make blades less visible to air traffic and air defence radars, Vestas can field a range of solutions, including the incorporation of stealth materials within the blade at the manufacturing stage. Which combination of measures is chosen depends on factors such as the importance of the radar concerned, the level of interference threatened by the proposed turbines, consultants' assessments of that threat, the strength of opposition to the wind farm proposal, the allocated budget and the costs and efficacy of remedial measures.

Location criteria critical

The location on the Isle of Wight for Vestas' new global R&D centre was chosen partly for its waterside access, a necessity for producers of the very large rotor blades needed for future offshore use. The quay enables blades to be transferred to and from purpose-built Bladerunner ships that can transport them across the Solent to Southampton Docks for onward shipment to other locations.

Another factor behind the choice of site was the concentration of composites expertise present on the Isle of Wight thanks to its broad sweep of aerospace, marine and renewable energy activities. Vestas already has a facility at Venture Quays in Cowes and, what is now a turbine component factory in Newport. Overall it employs some 220 people on the Isle of Wight, including the staff at the new R&D centre.

The South East England Development Agency (SEEDA), subsequently a casualty of Government cuts, injected £3 million to fund the upgrading of the main half-mile access road to the facility. The Isle of Wight County Council, keen to promote job creation around its ‘Eco Island' concept, was also highly supportive.

Green by design

Conceived five years ago and constructed over the last 18 months, Vestas' new global R&D centre was built by Sir Robert McAlpine Ltd to a ‘green' design evolved by the Danish turbine manufacturer and its architects.

Carefully planned glazed areas admit copious natural daylight so that the use of artificial lighting, although energy efficient, is minimised. Similarly, the building design encourages natural ventilation.

Ground source heat pumps, based on 50, 100 metre deep boreholes, provide heating while working temperatures within the halls and offices are controlled to comfortable, but not excessive, levels. Heat conservation is ensured by effective sealing of windows and doors along with high levels of insulation.

An artificial lake stores and partially cleans grey (used) water ready for utility re-use. Allotment plots located over the underground heat pump installation are in strong demand from employees wishing to grow vegetables and other produce.

Cycle commuting is encouraged by the provision of covered cycle racks, showers, personal cubicles and the government-assisted scheme that enables employees to acquire cycles and pay for them on favourable terms through automatic salary deductions. Up to 50 personnel at the Vestas facility cycle to work on any given day, many of them using a cycle route that borders the site and connects the conurbations of Newport and Cowes. Meanwhile, company-owned electric cycles enable staff to shuttle between the R&D facility and other Vestas sites, and there are also a small number of electric cars available.

In the next article we will look at a different approach to scaling up wind turbine rotors, advocated by Blade Dynamics Ltd, a young company located in the Isle of Wight, UK and New Orleans, USA.

About George Marsh: Engineering roles in high-vacuum physics, electronics, flight testing and radar led George Marsh, via technology PR, to technology journalism. He is a regular contributor to Renewable Energy Focus.

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